In order to gain a deeper insight into the genetic regulation of cytokine-determined and immunoglobulin/ T cell receptor based signaling in lymphocytes, efforts to use RNA interference (RNAi) technology as a screening tool have been undertaken. Selected libraries of shRNAs and siRNAs have been obtained. Introduction of siRNAs into resting CD4 T cells using Amaxa transfection technology has been achieved and, in test systems, impressive inhibition of expression and function of kinases such as Jak3 has been obtained. Currently, we have assembled a protein tyrosine kinase library and a protein tyrosine phosphatase library that will be tested for its capacity to inhibit or enhance the induction of Th2 phenotype by naive CD4 T cells. To this end, we have developed a "recombineered" mouse that faithfully expresses DS-Red as a surrogate for IL-13. Thus, cells can be tested for induction or inhibition of DS-Red expression and thus restimulation can be avoided. We anticipate both cloning cells that have been selected (i.e. either induced or suppressed, depending on the experimental protocol) and determining what shRNAs they have incorporated or using bulk induced or suppressed cells, carrying out microarray analysis of the shRNAs expressed by the bulk slected populations. As a test of overall feasibility of this approach, an initial effort at genome-wide siRNA screening has been performed with a mouse GeneNet lentiviral siRNA Library comprising 39,000 genes purchased from System Biosciences Inc. targeting the induction of CD23 in M12.4.1 B lymphoma cells by IL-4. The library was successfully introduced in M12.4.1 cells by infection and non-infected cells eliminated based on the resistance of the infected cells to puromycin. The infected M12 cells were then stimulated with IL-4 and the cells that expressed CD23 were separated from the CD23-negative cells by fluorescence-based cell sorting. The negative cells were restimulated; expressing and non-expressing cells were separated again. After a third round, the CD23+ and CD23- cells were harvested, the lentiviral siRNA inserts amplified by PCR using primers from the flanking sequences in the vector and the amplified sequences evaluated by microarray analysis using Affymetrix gene chips. A series of candidate sequences were found that were strikingly enriched in the CD23-negative cells, presumably reflecting those siRNAs more likely to be found in non-CD23-expressors and thus, by inference, that have played some role in preventing cells from expressing CD23. Among these are the genes for: Jak1, Map4k2(a Map kinase, knase, kinase kinase), syk and a non-receptor-type protein tyrosine phosphatase.